Some arc discharge lamps, such as metal halide lamps, contain fill materials generally including mercury, an inert gas and one or more metal halides which produce vapors that support a luminous arc discharge during operation. The presence of other materials, even in small quantities, contaminates the arc tube and can cause a reduction in light output, a change in color temperature or operating voltage and/or premature lamp failure. Contaminants can be introduced into the arc tube at various times during the manufacturing process. In particular, contaminants introduced during the electrode sealing process or remaining after electrodes are sealed into the lamp are difficult to remove. In the case of metal halide lamps, such contaminants include silica smoke (silica which has vaporized and then recondensed), tungsten oxides originating from the electrodes and water which adsorbed on surfaces inside the arc tube.
Various techniques are known for removing contaminants from arc tubes. Typically, a series of steps are used to remove contaminants, including washing the arc tube with hydrofluoric acid to remove silica smoke, baking the arc tube for 6-24 hours to volatilize and drive off tungsten oxides and water, repeated filling and exhausting with a gas, and striking an arc to clean the electrodes. These steps are relatively expensive and time-consuming and can potentially introduce additional contaminants if they are not carefully controlled.
In the fill and exhaust method of flushing an arc tube, a gas is introduced into the arc tube through the lamp tubulation and then is removed by a vacuum pumping through the tubulation. This procedure is repeated to reduce the contaminant level gradually. However, this technique is not particularly effective. Since the same tubulation is used for both fill and exhaust, contaminants are deposited on the tubulation wall during exhaust and are reintroduced into the arc tube during succeeding fill cycles.
A method for manufacturing metal halide lamps which utilizes an arc tube without a tubulation is disclosed by R. L. Hansler et al in "A New Low Wattage Metal Halide Process", IES Annual Conference, 1985, pages 109-122. In the Hansler et al process, argon is flushed through the arc tube from end to end during the process until the electrodes are sealed in the ends of the tube. During this flush, contaminants driven from the wall, etc., will be carried out. The Hansler et al process overcomes the problem of recontamination that arises from flush pumping, but does not remove contaminants which remain after the electrodes are sealed into the arc tube.
It is a general object of the present invention to provide improved methods for manufacturing arc tubes.
It is another object of the present invention to provide improved methods for removing contaminants from arc tubes.
It is a further object of the present invention to provide methods for removing contaminants from arc tubes after electrodes are sealed therein.
It is yet another object of the present invention to provide methods for removing contaminants from arc tubes, wherein contaminants are completely purged from the system and not available for reintroduction into the same lamp or different lamps.
It is a further object of the present invention to provide a method for manufacturing arc tubes in which contaminants may be removed without the utilization of vacuum pumps.
It is still another object of the present invention to provide methods for removing contaminants from arc tubes during and after a press sealing operation.
It is still another object of the present invention to provide methods for removing contaminants from arc tubes at atmospheric pressure or above.